1. Field
The present disclosure generally relates to the design of photonic integrated circuits (PICs). More specifically, the present disclosure relates to a PIC that includes a grating coupler that couples an optical signal to another PIC.
2. Related Art
Optical interconnects or links based on silicon photonics have the potential to alleviate inter-chip communication bottlenecks in high-performance computing systems that include multiple processor chips and memory chips. This is because, relative to electrical interconnects, optical interconnects offer significantly improved: bandwidth, density, power consumption, latency, and range.
In order to make a very low power (for example, less than 1 pJ/bit) optical interconnect, a high-efficiency optical source, such as a semiconductor laser or a laser source, is typically required. In particular, the required optical-waveguide-coupled wall-plug efficiency (defined as the laser power coupled into a silicon optical waveguide divided by the total consumed electrical power) of such a laser source usually needs to be greater than 10%. In addition, if silicon-photonic resonator devices (such as ring modulators) are used in an optical interconnect, the spectral linewidth of the laser source may need to be less than 10 μm.
However, most state-of-the-art laser sources have a wall-plug efficiency of only 1-2%. In these laser sources, a large amount (in excess of 80%) of the electrical power is usually consumed by thermal-electric cooling (TEC) to maintain high-power (greater than 10 mW) lasing with stable wavelength and good slope efficiency. While uncooled laser sources with sufficient wall-plug efficiency (around 10%) and output power (for example, 2-4 mW) are available for use in optical interconnects, the wavelength stability of these laser sources is often larger than 100 μm (because of the lack of temperature control). In addition, these laser sources are usually based on III-V semiconductors (such as indium phosphide, etc.). The large optical coupling loss between an optical waveguide in the III-V semiconductor laser source and a silicon optical waveguide could reduce the efficiency by 3-10 times.
Hence, what is needed is an optical source without the problems described above.